Polybenzoxazole membranes from self-cross-linkable aromatic polyimide membranes

ABSTRACT

A method of making a polybenzoxazole (PBO) membrane from a self-cross-linked aromatic polyimide polymer membrane is provided. These membranes are useful in the separation of gas mixtures and liquid mixtures. The PBO membrane is made by fabricating a self-cross-linkable aromatic polyimide polymer membrane comprising both hydroxyl functional groups and carboxylic acid functional groups; cross-linking the polymer to form a self-cross-linked aromatic polyimide polymer membrane by heating the membrane at 250° to 300° C. under an inert atmosphere; and thermal heating the self-cross-linked aromatic polyimide polymer membrane at a temperature from about 350° to 500° C. under an inert atmosphere to convert the self-cross-linked aromatic polyimide polymer membrane into a PBO membrane. A membrane coating step may be added by coating the selective layer surface of the PBO membrane with a thin layer of high permeability material.

BACKGROUND OF THE INVENTION

This invention relates to a method of making polybenzoxazole (PBO)membranes from self-cross-linkable aromatic polyimide polymer comprisingboth hydroxyl functional groups and carboxylic acid functional groupsand the use of these membranes in separations of gas mixtures and liquidmixtures.

In the past 30-35 years, the state of the art of polymer membrane-basedgas separation processes has evolved rapidly. Membrane-basedtechnologies have advantages of both low capital cost and high-energyefficiency compared to conventional separation methods. Membrane gasseparation is of special interest to petroleum producers and refiners,chemical companies, and industrial gas suppliers. Several applicationshave achieved commercial success, including carbon dioxide removal fromnatural gas and from biogas and enhanced oil recovery, and also inhydrogen removal from nitrogen, methane, and argon in ammonia purge gasstreams. For example, UOP's Separex™ cellulose acetate polymericmembrane is currently an international market leader for carbon dioxideremoval from natural gas.

Cellulose acetate (CA) glassy polymer membranes are used extensively ingas separation. Currently, such CA membranes are used commercially fornatural gas upgrading, including the removal of carbon dioxide. AlthoughCA membranes have many advantages, they are limited in a number ofproperties including selectivity, permeability, and in chemical,thermal, and mechanical stability. It has been found that polymermembrane performance can deteriorate quickly. A primary cause of loss ofmembrane performance is liquid condensation on the membrane surface.Condensation can be prevented by providing a sufficient dew point marginfor operation, based on the calculated dew point of the membrane productgas. UOP's MemGuard™ system, a regenerable adsorbent system that usesmolecular sieves, was developed to remove water as well as heavyhydrocarbons from the natural gas stream, hence, to lower the dew pointof the stream. The selective removal of heavy hydrocarbons by apretreatment system can significantly improve the performance of themembranes. Although these pretreatment systems can effectively performthis function, the cost is quite significant. In some projects, the costof the pretreatment system was as high as 10 to 40% of the total cost(pretreatment system and membrane system) depending on the feedcomposition. Reduction of the size of the pretreatment system or eventotal elimination of the pretreatment system would significantly reducethe membrane system cost for natural gas upgrading. Another factor isthat, in recent years, more and more membrane systems have beeninstalled in large offshore natural gas upgrading projects. Thefootprint is a big constraint for offshore projects. The footprint ofthe pretreatment system is very high at more than 10 to 50% of thefootprint of the entire membrane system. Therefore, removal of thepretreatment system from the membrane system has great economic impact,especially to offshore projects.

Aromatic polybenzoxazoles (PBOs), polybenzthiazoles (PBTs), andpolybenzimidazoles (PBIs) are thermally stable ladder-like glassypolymers with flat, stiff, rigid-rod phenylene-heterocyclic ring units.The stiff, rigid ring units in such polymers pack efficiently, leavingvery small penetrant-accessible free volume elements that are desirableto provide polymer membranes with both high permeability and highselectivity. These aromatic PBO, PBT, and PBI polymers, however, havepoor solubility in common organic solvents, preventing them from beingused for making polymer membranes by the most practical solvent castingmethod.

Thermal conversion of soluble aromatic polyimides containing pendentfunctional groups ortho to the heterocyclic imide nitrogen in thepolymer backbone to aromatic polybenzoxazoles (PBOs) orpolybenzthiazoles (PBTs) has been found to provide an alternative methodfor creating PBO or PBT polymer membranes that are difficult orimpossible to obtain directly from PBO or PBT polymers by solventcasting (Tullos et al, MACROMOLECULES, 32, 3598 (1999)). A recentpublication in the journal SCIENCE reported high permeabilitypolybenzoxazole polymer membranes in dense film geometry for gasseparations (Ho Bum Park et al, SCIENCE 318, 254 (2007)). Thesepolybenzoxazole membranes are prepared from high temperature thermalrearrangement of hydroxy-containing polyimide polymer membranescontaining pendent hydroxyl groups ortho to the heterocyclic imidenitrogen. These polybenzoxazole polymer membranes exhibited extremelyhigh CO₂ permeability (>100 Barrer) which is at least 10 times betterthan conventional polymer membranes.

Poly(o-hydroxy amide) polymers comprising pendent phenolic hydroxylgroups ortho to the amide nitrogen in the polymer backbone have alsobeen used for making PBO membranes for separation applications (US2010/0133188 A1).

One of the components to be separated by a membrane must have asufficiently high permeance at preferred conditions or extraordinarilylarge membrane surface areas are required to allow separation of largeamounts of material. Permeance, measured in Gas Permeation Units (GPU, 1GPU=7.5×10⁻⁹ m³ (STP)/m² s (kPa)), is the pressure normalized flux andis equal to permeability divided by the skin layer thickness of themembrane. Commercially available polymer membranes, such as celluloseacetate and polysulfone membranes, have an asymmetric structure with athin dense selective layer of less than 1 μm. The thin selective layerprovides the membrane high permeance representing high productivity.Therefore, it is highly desirable to prepare asymmetric PBO membraneswith high permeance for separation applications. One such type ofasymmetric hollow fiber PBO membrane has been recently disclosed by Parket al. (US 2009/0297850 A1) and Visser et al. (Abstract on “Developmentof asymmetric hollow fiber membranes with tunable gas separationproperties” at NAMS 2009 conference, Jun. 20-24, 2009, Charleston, S.C.,USA). The asymmetric hollow fiber PBO membranes disclosed by Park et al.and Visser et al. were obtained from o-hydroxyl substituted polyimideasymmetric hollow fiber membranes via thermal rearrangement. However,Visser et al. found that the high temperature thermally rearrangedasymmetric hollow fiber PBO membranes had low gas permeances (equivalentto a dense selective layer thickness of >5 μm). The low gas permeance isbecause the fiber shrank and the porous substructure collapsed duringthermal rearrangement at temperatures higher than 300° C. Therefore,much more research is still required to reduce the excessivedensification of the porous membrane substructure of asymmetrico-hydroxyl substituted polyimide membranes during thermal rearrangementat elevated temperature to make asymmetric PBO membranes.

Park et al. also disclosed asymmetric hollow fiber PBO membranesobtained from o-hydroxyl substituted polyamic acid asymmetric hollowfiber membranes via thermal rearrangement (WO 2009142433 and US2009/0282982 A1).

The present invention provides a method of making polybenzoxazole (PBO)membranes from self-cross-linkable aromatic polyimide polymer comprisingboth hydroxyl functional groups and carboxylic acid functional groupsand methods of using these membranes.

DETAILED DESCRIPTION OF THE INVENTION

The present invention generally relates to a method of making PBOmembranes from self-cross-linkable aromatic polyimide polymer comprisingboth hydroxyl functional groups and carboxylic acid functional groups.The present invention also relates to the use of PBO membranes for gas,vapor, and liquid separations.

The present invention provides a method for the production of a PBOmembrane by: 1) fabricating a self-cross-linkable aromatic polyimidepolymer membrane from the self-cross-linkable aromatic polyimide polymercomprising both hydroxyl functional groups and carboxylic acidfunctional groups; 2) cross-linking the self-cross-linkable aromaticpolyimide polymer membrane to form the self-cross-linked aromaticpolyimide polymer membrane by heating the membrane at 250° C. to 300° C.under an inert atmosphere, such as argon, nitrogen, or vacuum; 3)thermal heating the self-cross-linked aromatic polyimide polymermembrane at a temperature from about 350° to 500° C. under an inertatmosphere, such as argon, nitrogen, or vacuum to convert theself-cross-linked aromatic polyimide polymer membrane into a PBOmembrane. In some cases, a membrane coating step is added after step 3)by coating the selective layer surface of the PBO membrane with a thinlayer of high permeability material such as a polysiloxane, afluoro-polymer, a thermally curable silicone rubber, or a UV radiationcurable epoxy silicone.

The advantage of using self-cross-linkable aromatic polyimide polymercomprising both hydroxyl functional groups and carboxylic acidfunctional groups to prepare PBO membrane in the present invention is toprevent the densification of skin layer and substructure during PBOconversion at a temperature between 350° to 500° C. Skin layer andsubstructure collapse during PBO conversion from traditional aromaticpolyimide polymer comprising hydroxyl functional groups and withoutcarboxylic acid functional groups at a temperature between 350° to 500°C. resulted in significantly increased effective separation layerthickness and therefore significantly reduced membrane permeance. Theglass-rubber transition temperature (Tg) of the traditional aromaticpolyimide polymer comprising hydroxyl functional groups and withoutcarboxylic acid functional groups is below the PBO conversiontemperature, which will result in substructure collapse. However, theself-cross-linked aromatic polyimide polymer described in the presentinvention has a T_(g) well above its decomposition temperature. Theformation of the self-cross-linked aromatic polyimide polymer membranein the present invention via heating the self-cross-linkable aromaticpolyimide polymer membrane at <300° C., which is below the T_(g) of theself-cross-linkable aromatic polyimide polymer, prevents thedensification of skin layer and substructure during PBO conversion at atemperature between 350° to 500° C.

The term “self-cross-linkable aromatic polyimide polymer” in the presentinvention refers to an aromatic polyimide polymer comprising bothcarboxylic acid functional groups and hydroxyl functional groups whereinthe carboxylic acid functional groups can react with the hydroxylfunctional groups via heating. The term “self-cross-linked aromaticpolyimide polymer membrane” in the present invention refers to anaromatic polyimide polymer membrane comprising self-cross-linkedaromatic polyimide polymer that comprises covalent ester bonds formedfrom esterification reaction between carboxylic acid functional groupsand hydroxyl functional groups.

The self-cross-linkable aromatic polyimide polymer used for thepreparation of PBO membrane described in the present invention comprisesboth hydroxyl functional groups and carboxylic acid functional groups.The self-cross-linkable aromatic polyimide polymer and theself-cross-linkable aromatic polyimide polymer membrane used for thepreparation of PBO membrane described in the present invention comprisea plurality of repeating units of formula (I), wherein formula (I)comprises carboxylic acid functional groups and hydroxyl functionalgroups, and wherein the carboxylic acid functional groups can react withthe hydroxyl functional groups via covalent ester bonds at 250° to 300°C. to form self-cross-linked aromatic polyimide polymer described in thepresent invention comprising a plurality of repeating units of formula(II). The self-cross-linked aromatic polyimide polymer and theself-cross-linked aromatic polyimide polymer membrane used for thepreparation of PBO membrane described in the present invention comprisearomatic polyimide polymer chain segments where at least part of thesepolymer chain segments are cross-linked to each other through directcovalent ester bonds. The formation of the covalent ester bonds amongthe aromatic polyimide polymer chains via the self-cross-linking of theself-cross-linkable aromatic polyimide polymer comprising bothcarboxylic acid functional groups and hydroxyl functional groups at 250°to 300° C. results in self-cross-linked aromatic polyimide polymer witha T_(g) well above its decomposition temperature. The self-cross-linkedaromatic polyimide polymer membrane is converted into an a PBO membraneby thermal rearrangement at a temperature from about 350° to 500° C.under an inert atmosphere, such as argon, nitrogen, or vacuum. Theheating time for this heating step is in a range of about 30 seconds to2 hours. A more preferred heating time is from about 30 seconds to 1hour. The PBO membrane prepared from the self-cross-linkable aromaticpolyimide polymer membrane described in the present invention showedsignificantly higher permeability than the self-cross-linkable aromaticpolyimide polymer membrane and the self-cross-linked aromatic polyimidepolymer membrane for a variety of gas separation applications such asCO₂/CH₄, H₂/CH₄, and He/CH₄ separations. For example, a PBO membraneprepared from the self-cross-linkablepoly[2,2′-bis-(3,4-dicarboxyphenyl)hexafluoropropanedianhydride-3,3′-dihydroxy-4,4′-diamino-biphenyl-3,5-diaminobenzoicacid]polyimide (abbreviated as poly(6FDA-HAB-DBA)) membrane via heatingat 450° C. has a high CO₂ permeance of 210 Barrers and CO₂/CH₄selectivity of 25.9 for CO₂/CH₄ separation. This PBO membrane also has ahigh H₂ permeance of 337.1 Barrers and H₂/CH₄ selectivity of 41.5 forH₂/CH₄ separation.

The self-cross-linkable aromatic polyimide polymer used for thepreparation of PBO membrane described in the present invention comprisesa formula (I):

wherein X₁ and X₂ are selected from the group consisting of

and mixtures thereof, respectively; X₁ and X₂ are the same or differentfrom each other; Y₁—COOH is selected from the group consisting of

and mixtures thereof; Y₂—OR is selected from the group consisting of

and mixtures thereof, and —R is selected from the group consisting of —Hand a mixture of —H and —COCH₃, and —R′— is selected from the groupconsisting of

and mixtures thereof; n and m are independent integers from 2 to 500;the molar ratio of n/m is in a range of 1:1 to 1:20.

The self-cross-linkable aromatic polyimide polymer comprising bothhydroxyl functional groups and carboxylic acid functional groups usedfor the preparation of PBO membrane of the invention may be selectedfrom the group consisting of poly(3,3′,4,4′-diphenylsulfonetetracarboxylic dianhydride-3,5-diaminobenzoicacid-3,3′-dihydroxy-4,4′-diamino-biphenyl)polyimide derived from apolycondensation reaction of 3,3′,4,4′-diphenylsulfone tetracarboxylicdianhydride with a mixture of 3,5-diaminobenzoic acid and3,3′-dihydroxy-4,4′-diamino-biphenyl; poly(3,3′,4,4′-benzophenonetetracarboxylic dianhydride-pyromellitic dianhydride-3,5-diaminobenzoicacid-3,3′-dihydroxy-4,4′-diamino-biphenyl)polyimide derived from apolycondensation reaction of 3,3′,4,4′-benzophenone tetracarboxylicdianhydride and pyromellitic dianhydride with 3,5-diaminobenzoic acidand 3,3′-dihydroxy-4,4′-diamino-biphenyl; poly(3,3′,4,4′-benzophenonetetracarboxylic dianhydride-3,5-diaminobenzoicacid-3,3′-dihydroxy-4,4′-diamino-biphenyl)polyimide derived from apolycondensation reaction of 3,3′,4,4′-benzophenone tetracarboxylicdianhydride with 3,5-diaminobenzoic acid and3,3′-dihydroxy-4,4′-diamino-biphenyl;poly[2,2′-bis-(3,4-dicarboxyphenyl)hexafluoropropanedianhydride-3,5-diaminobenzoicacid-3,3′-dihydroxy-4,4′-diamino-biphenyl]polyimide derived from thepolycondensation reaction of2,2′-bis-(3,4-dicarboxyphenyl)hexafluoropropane dianhydride with amixture of 3,5-diaminobenzoic acid and3,3′-dihydroxy-4,4′-diamino-biphenyl;poly[2,2′-bis-(3,4-dicarboxyphenyl)hexafluoropropanedianhydride-2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane-3,5-diaminobenzoicacid] derived from a polycondensation reaction of2,2′-bis-(3,4-dicarboxyphenyl)hexafluoropropane dianhydride with amixture of 2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane and3,5-diaminobenzoic acid; poly[3,3′,4,4′-benzophenonetetracarboxylicdianhydride-2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane-3,5-diaminobenzoicacid] derived from a polycondensation reaction of3,3′,4,4′-benzophenonetetracarboxylic dianhydride with a mixture of2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane and3,5-diaminobenzoic acid; poly[4,4′-oxydiphthalicanhydride-2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane-3,5-diaminobenzoicacid] derived from a polycondensation reaction of 4,4′-oxydiphthalicanhydride with a mixture of2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane and3,5-diaminobenzoic acid; poly[3,3′,4,4′-diphenylsulfone tetracarboxylicdianhydride-2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane-3,5-diaminobenzoicacid] derived from a polycondensation reaction of3,3′,4,4′-diphenylsulfone tetracarboxylic dianhydride with a mixture of2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane and3,5-diaminobenzoic acid;poly[2,2′-bis-(3,4-dicarboxyphenyl)hexafluoropropanedianhydride-3,3′,4,4′-benzophenonetetracarboxylicdianhydride-2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane-3,5-diaminobenzoicacid] derived from a polycondensation reaction of2,2′-bis-(3,4-dicarboxyphenyl)hexafluoropropane dianhydride and3,3′,4,4′-benzophenonetetracarboxylic dianhydride with a mixture of2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane and3,5-diaminobenzoic acid; poly[4,4′-oxydiphthalicanhydride-2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane-3,3′-dihydroxy-4,4′-diamino-biphenyl-3,5-diaminobenzoicacid] derived from a polycondensation reaction of 4,4′-oxydiphthalicanhydride with a mixture of2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane,3,3′-dihydroxy-4,4′-diamino-biphenyl and 3,5-diaminobenzoic acid;poly[3,3′,4,4′-benzophenonetetracarboxylicdianhydride-2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane-3,3′-dihydroxy-4,4′-diamino-biphenyl-3,5-diaminobenzoicacid] derived from a polycondensation reaction of3,3′,4,4′-benzophenonetetracarboxylic dianhydride with a mixture of2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane,3,3′-dihydroxy-4,4′-diamino-biphenyl, and 3,5-diaminobenzoic acid.

The self-cross-linked aromatic polyimide polymer formed from theself-cross-linkable aromatic polyimide polymer described in the presentinvention comprises a plurality of repeating units of formula (II):

wherein X₁ and X₂ are selected from the group consisting of

and mixtures thereof, respectively; X₁ and X₂ are the same or differentfrom each other; Y₁—CO— is selected from the group consisting of

and mixtures thereof; Y₂—O— is selected from the group consisting of

and mixtures thereof, and —R′— is selected from the group consisting of

and mixtures thereof; Y₂—OR is selected from the group consisting of

and mixtures thereof, and —R— is selected from the group consisting of—H, and a mixture of —H and —COCH₃, and —R′— is selected from the groupconsisting of

and mixtures thereof; n′, n″, m′, m″, p, and p′ are independent integersfrom 2 to 500; the molar ratio of n′/(m′+p) is in a range of 1:1 to1:20; the molar ratio of n″/(m″+p′) is in a range of 1:1 to 1:20.

The self-cross-linkable aromatic polyimide polymer used for thepreparation of PBO membrane described in the present invention has aweight average molecular weight in the range of 10,000 to 1,000,000Daltons, preferably between 70,000 to 500,000 Daltons.

The polybenzoxazole polymer in the polybenzoxazole membrane made fromthe self-cross-linkable aromatic polyimide polymer in the presentinvention comprises the repeating units of a formula (III), wherein saidformula (III) is:

wherein X₁ is selected from the group consisting of

and mixtures thereof; wherein X₃ is selected from the group consistingof

and mixtures thereof; wherein X₄ is selected from the group consistingof

and mixtures thereof; Y₁ is selected from the group consisting of

and mixtures thereof; Y₂ is selected from the group consisting of

and mixtures thereof, and —R′— is selected from the group consisting of

and mixtures thereof; o and q are independent integers from 2 to 500.

In some cases a membrane post-treatment step can be added after theformation of the PBO polymer membrane with the application of a thinlayer of a high permeability material such as a polysiloxane, afluoro-polymer, a thermally curable silicone rubber, or a UV radiationcurable epoxy silicone. The coating fills the surface pores and otherimperfections comprising voids (see U.S. Pat. No. 4,230,463; U.S. Pat.No. 4,877,528; and U.S. Pat. No. 6,368,382).

The self-cross-linkable aromatic polyimide polymer membrane and the PBOmembrane made from the self-cross-linkable aromatic polyimide polymerdescribed in the present invention can be fabricated into any convenientgeometry such as flat sheet (or spiral wound), tube, or hollow fiber.

The invention provides a process for separating at least one gas from amixture of gases using the PBO membrane made from the self-cross-linkedaromatic polyimide polymer membrane described in the present invention,the process comprising: (a) providing a PBO membrane made from theself-cross-linked aromatic polyimide polymer membrane described in thepresent invention which is permeable to said at least one gas; (b)contacting the mixture on one side of the PBO membrane made from theself-cross-linked aromatic polyimide polymer membrane described in thepresent invention to cause said at least one gas to permeate themembrane; and (c) removing from the opposite side of the membrane apermeate gas composition comprising a portion of said at least one gaswhich permeated said membrane.

The PBO membrane made from the self-cross-linked aromatic polyimidepolymer membrane described in the present invention is especially usefulin the purification, separation or adsorption of a particular species inthe liquid or gas phase. In addition to separation of pairs of gases,the PBO membrane made from the self-cross-linked aromatic polyimidepolymer membrane described in the present invention may, for example, beused for the desalination of water by reverse osmosis or for theseparation of proteins or other thermally unstable compounds, e.g. inthe pharmaceutical and biotechnology industries. The PBO membrane madefrom the self-cross-linked aromatic polyimide polymer membrane describedin the present invention may also be used in fermenters and bioreactorsto transport gases into the reaction vessel and transfer cell culturemedium out of the vessel. Additionally, the PBO membrane made from theself-cross-linked aromatic polyimide polymer membrane described in thepresent invention may be used for the removal of microorganisms from airor water streams, water purification, ethanol production in a continuousfermentation/membrane pervaporation system, and in detection or removalof trace compounds or metal salts in air or water streams.

The PBO membrane made from the self-cross-linked aromatic polyimidepolymer membrane described in the present invention is especially usefulin gas separation processes in air purification, petrochemical,refinery, and natural gas industries. Examples of such separationsinclude separation of volatile organic compounds (such as toluene,xylene, and acetone) from an atmospheric gas, such as nitrogen or oxygenand nitrogen recovery from air. Further examples of such separations arefor the separation of He, CO₂ or H₂S from natural gas, H₂ from N₂, CH₄,and Ar in ammonia purge gas streams, H₂ recovery in refineries,olefin/paraffin separations such as propylene/propane separation, xyleneseparations, iso/normal paraffin separations, liquid natural gasseparations, C₂+ hydrocarbon recovery. Any given pair or group of gasesthat differ in molecular size, for example nitrogen and oxygen, carbondioxide and methane, hydrogen and methane or carbon monoxide, helium andmethane, can be separated using the PBO membrane made from theself-cross-linked aromatic polyimide polymer membrane described in thepresent invention. More than two gases can be removed from a third gas.For example, some of the gas components which can be selectively removedfrom a raw natural gas using the PBO membrane made from theself-cross-linked aromatic polyimide polymer membrane described hereininclude carbon dioxide, oxygen, nitrogen, water vapor, hydrogen sulfide,helium, and other trace gases. Some of the gas components that can beselectively retained include hydrocarbon gases. When permeablecomponents are acid components selected from the group consisting ofcarbon dioxide, hydrogen sulfide, and mixtures thereof and are removedfrom a hydrocarbon mixture such as natural gas, one module, or at leasttwo in parallel service, or a series of modules may be utilized toremove the acid components. For example, when one module is utilized,the pressure of the feed gas may vary from 275 kPa to about 2.6 MPa (25to 4000 psi). The differential pressure across the membrane can be aslow as about 70 kPa or as high as 14.5 MPa (about 10 psi or as high asabout 2100 psi) depending on many factors such as the particularmembrane used, the flow rate of the inlet stream and the availability ofa compressor to compress the permeate stream if such compression isdesired. Differential pressure greater than about 14.5 MPa (2100 psi)may rupture the membrane. A differential pressure of at least 0.7 MPa(100 psi) is preferred since lower differential pressures may requiremore modules, more time and compression of intermediate product streams.The operating temperature of the process may vary depending upon thetemperature of the feed stream and upon ambient temperature conditions.Preferably, the effective operating temperature of the membranes of thepresent invention will range from about −50° to about 150° C. Morepreferably, the effective operating temperature of the PBO membrane madefrom the self-cross-linked aromatic polyimide polymer membrane of thepresent invention will range from about −20° to about 100° C., and mostpreferably, the effective operating temperature of the membranes of thepresent invention will range from about 25° to about 100° C.

The PBO membrane made from the self-cross-linked aromatic polyimidepolymer membrane described in the present invention are also especiallyuseful in gas/vapor separation processes in chemical, petrochemical,pharmaceutical and allied industries for removing organic vapors fromgas streams, e.g. in off-gas treatment for recovery of volatile organiccompounds to meet clean air regulations, or within process streams inproduction plants so that valuable compounds (e.g., vinylchloridemonomer, propylene) may be recovered. Further examples of gas/vaporseparation processes in which the PBO membrane made from theself-cross-linked aromatic polyimide polymer membrane described in thepresent invention may be used are hydrocarbon vapor separation fromhydrogen in oil and gas refineries, for hydrocarbon dew pointing ofnatural gas (i.e. to decrease the hydrocarbon dew point to below thelowest possible export pipeline temperature so that liquid hydrocarbonsdo not separate in the pipeline), for control of methane number in fuelgas for gas engines and gas turbines, and for gasoline recovery. The PBOmembrane made from the self-cross-linked aromatic polyimide polymermembrane described in the present invention may incorporate a speciesthat adsorbs strongly to certain gases (e.g. cobalt porphyrins orphthalocyanines for O₂ or silver (I) for ethane) to facilitate theirtransport across the membrane.

The PBO membrane made from the self-cross-linked aromatic polyimidepolymer membrane described in the present invention also has immediateapplication to concentrate olefin in a paraffin/olefin stream for olefincracking application. For example, the PBO membrane made from theself-cross-linked aromatic polyimide polymer membrane described in thepresent invention can be used for propylene/propane separation toincrease the concentration of the effluent in a catalyticdehydrogenation reaction for the production of propylene from propaneand isobutylene from isobutane. Therefore, the number of stages of apropylene/propane splitter that is required to get polymer gradepropylene can be reduced. Another application for the PBO membrane madefrom the self-cross-linked aromatic polyimide polymer membrane describedin the present invention is for separating isoparaffin and normalparaffin in light paraffin isomerization and MaxEne™, a process forenhancing the concentration of normal paraffin (n-paraffin) in thenaphtha cracker feedstock, which can be then converted to ethylene.

The PBO membrane made from the self-cross-linked aromatic polyimidepolymer membrane described in the present invention can also be operatedat high temperature to provide the sufficient dew point margin fornatural gas upgrading (e.g, CO₂ removal from natural gas). The PBOmembrane made from the self-cross-linked aromatic polyimide polymermembrane described in the present invention can be used in either asingle stage membrane or as the first or/and second stage membrane in atwo stage membrane system for natural gas upgrading.

The PBO membrane made from the self-cross-linked aromatic polyimidepolymer membrane described in the present invention may also be used inthe separation of liquid mixtures by pervaporation, such as in theremoval of organic compounds (e.g., alcohols, phenols, chlorinatedhydrocarbons, pyridines, ketones) from water such as aqueous effluentsor process fluids. A membrane which is ethanol-selective would be usedto increase the ethanol concentration in relatively dilute ethanolsolutions (5-10% ethanol) obtained by fermentation processes. Anotherliquid phase separation example using the PBO membrane made from theself-cross-linked aromatic polyimide polymer membrane described in thepresent invention is the deep desulfurization of gasoline and dieselfuels by a pervaporation membrane process similar to the processdescribed in U.S. Pat. No. 7,048,846, incorporated by reference hereinin its entirety. The PBO membrane made from the self-cross-linkedaromatic polyimide polymer membrane described in the present inventionthat are selective to sulfur-containing molecules would be used toselectively remove sulfur-containing molecules from fluid catalyticcracking (FCC) and other naphtha hydrocarbon streams. Further liquidphase examples include the separation of one organic component fromanother organic component, e.g. to separate isomers of organiccompounds. Mixtures of organic compounds which may be separated usingthe PBO membrane made from the self-cross-linked aromatic polyimidepolymer membrane described in the present invention include:ethylacetate-ethanol, diethylether-ethanol, acetic acid-ethanol,benzene-ethanol, chloroform-ethanol, chloroform-methanol,acetone-isopropylether, allylalcohol-allylether,allylalcohol-cyclohexane, butanol-butylacetate, butanol-1-butylether,ethanol-ethylbutylether, propylacetate-propanol,isopropylether-isopropanol, methanol-ethanol-isopropanol, andethylacetate-ethanol-acetic acid.

EXAMPLES

The following examples are provided to illustrate one or more preferredembodiments of the invention, but are not limited embodiments thereof.Numerous variations can be made to the following examples that liewithin the scope of the invention.

Example 1 Synthesis of Self-Cross-Linkable Aromatic Polyimidepoly[2,2′-bis-(3,4-dicarboxyphenyl)hexafluoropropanedianhydride-3,5-diaminobenzoicacid-3,3′-dihydroxy-4,4′-diamino-biphenyl] (Abbreviated aspoly(6FDA-HAB-DBA))

Poly(6FDA-HAB-DBA) polyimide was synthesized from polycondensationreaction of 2,2′-bis-(3,4-dicarboxyphenyl)hexafluoropropane dianhydride(6FDA) with a mixture of 3,5-diaminobenzoic acid (DBA) and3,3′-dihydroxy-4,4′-diamino-biphenyl (HAB) in DMAc polar solvent by atwo-step process involving the formation of the poly(amic acid) followedby a solution imidization process. Acetic anhydride was used as thedehydrating agent and pyridine was used as the imidization catalyst forthe solution imidization reaction. For example, a 1 L three-neckround-bottom flask equipped with a nitrogen inlet and a mechanicalstirrer was charged with 25.9 g (0.12 mol) of HAB, 4.56 g (0.03 mol) ofDBA and 121.8 g of DMAc. Once HAB and DBA were fully dissolved, 66.6 gof 6FDA (0.15 mol) was added to the HAB and DBA solution in the flask.Then 428 g of DMAc was added to the solution. The reaction mixture wasmechanically stirred for 24 hours at ambient temperature to give aviscous poly(amic acid) solution. Then 32.1 g (0.315 mol) of aceticanhydride and 49.8 g (0.63 mol) of pyridine were added to the reactionmixture under stirring. The reaction mixture was mechanically stirredfor an additional 3 hour at 95° C. to yield the poly(6FDA-HAB-DBA)polyimide. The poly(6FDA-HAB-DBA) polyimide product in a power form wasrecovered by adding methanol to the reaction mixture under mechanicalstirring. The resultant poly(6FDA-HAB-DBA) polyimide powder was thenthoroughly rinsed with methanol and dried in a vacuum oven at 110° C.for 24 hours.

Example 2 Preparation and Evaluation of PBO Membrane fromSelf-Cross-Linked Aromatic Polyimide Membrane

5.0 g of self-cross-linkable poly(6FDA-HAB-DBA) polyimide synthesized inExample 1 was dissolved in 20.0 g of NMP solvent. The mixture wasmechanically stirred for 2 hours to form a homogeneous casting dope. Theresulting homogeneous casting dope was allowed to degas overnight. Theself-cross-linkable poly(6FDA-HAB-DBA) membrane was prepared from thebubble free casting dope on a clean glass plate using a doctor knifewith a 15-mil gap. The membrane together with the glass plate was thenput into a vacuum oven. The solvents were removed by slowly increasingthe vacuum and the temperature of the vacuum oven. Finally, the membranewas heated at 200° C. under vacuum for 48 hours to completely remove theresidual solvents. The dried self-cross-linkable poly(6FDA-HAB-DBA)membrane was heated at 300° C. under N₂ for 10 min to form theself-cross-linked poly(6FDA-HAB-DBA) membrane via esterificationreaction between the carboxylic acid groups and the hydroxyl groups onpoly(6FDA-HAB-DBA) polymer chains. The self-cross-linkedpoly(6FDA-HAB-DBA) aromatic polyimide membrane became insoluble inorganic solvents.

The self-cross-linked poly(6FDA-HAB-DBA) membrane was then thermallyrearranged by heating from 60° to 450° C. at a heating rate of 15°C./min in a regular tube furnace under N₂ flow. The membrane was heldfor 10 min at 450° C. and then cooled down to 50° C. at a cooling rateof 15° C./min under N₂ flow to yield PBO(6FDA-HAB-DBA) membrane.

The PBO(6FDA-HAB-DBA) membrane made from the self-cross-linkedpoly(6FDA-HAB-DBA) aromatic polyimide membrane is useful for a varietyof gas separation applications such as CO₂/CH₄, H₂/CH₄, and He/CH₄separations. The membrane was tested for CO₂/CH₄ and H₂/CH₄ separationsat 50° C. under 791 kPa (100 psig) pure single feed gas pressure. Theresults show that the self-cross-linked poly(6FDA-HAB-DBA) aromaticpolyimide membrane has CO₂ permeance of 7.77 Barrers and CO₂/CH₄selectivity of 52.5 for CO₂/CH₄ separation. The PBO(6FDA-HAB-DBA)membrane made from the self-cross-linked poly(6FDA-HAB-DBA) aromaticpolyimide membrane showed significantly improved CO₂ permeance comparedto the self-cross-linked poly(6FDA-HAB-DBA) aromatic polyimide membranefor CO₂/CH₄ separation (Table 1). The PBO(6FDA-HAB-DBA) membrane madefrom the self-cross-linked poly(6FDA-HAB-DBA) aromatic polyimidemembrane also showed significantly improved H₂ permeance compared to theself-cross-linked poly(6FDA-HAB-DBA) aromatic polyimide membrane forH₂/CH₄ separation (Table 2).

TABLE 1 Pure gas permeation test results of self-cross-linkablepoly(6FDA-HAB-DBA) membrane, self-cross-linked poly(6FDA-HAB-DBA)membrane, and PBO(6FDA-HAB-DBA) membrane for CO₂/CH₄ Separation^(a)Membrane P_(CO2) (Barrer) α_(CO2/CH4) Self-cross-linkablepoly(6FDA-HAB-DBA) 5.13 49.3 Self-cross-linked poly(6FDA-HAB-DBA) 7.7752.5 PBO(6FDA-HAB-DBA) 209.9 25.9 ^(a)P_(CO2) and P_(CH4) were tested at50° C. and 690 kPa (100 psig); 1 Barrer = 10⁻¹⁰ cm³ (STP) · cm/cm² · sec· cmHg.

TABLE 2 Pure gas permeation test results of self-cross-linkablepoly(6FDA-HAB-DBA) membrane, self-cross-linked poly(6FDA-HAB-DBA)membrane, and PBO(6FDA-HAB-DBA) membrane for H₂/CH₄ Separation^(a)Membrane P_(H2) (Barrer) α_(H2/CH4) Self-cross-linkablepoly(6FDA-HAB-DBA) 22.9 220.1 Self-cross-linked poly(6FDA-HAB-DBA) 38.5260.3 PBO(6FDA-HAB-DBA) 337.1 41.5 ^(a)P_(H2) and P_(CH4) were tested at50° C. and 690 kPa (100 psig); Barrer = 10⁻¹⁰ cm³ (STP) · cm/cm² · sec ·cmHg

An embodiment of the invention involves a method of making apolybenzoxazole membrane comprising (a) fabricating aself-cross-linkable aromatic polyimide polymer membrane from theself-cross-linkable aromatic polyimide polymer comprising both hydroxylfunctional groups and carboxylic acid functional groups; (b)cross-linking the self-cross-linkable aromatic polyimide polymermembrane to form a self-cross-linked aromatic polyimide polymer membraneby heating the membrane at 250° C. to 300° C. under an inert atmosphere;and (c) thermal heating the self-cross-linked aromatic polyimide polymermembrane at a temperature from about 350° to 500° C. under an inertatmosphere, such as argon, nitrogen, or vacuum to convert theself-cross-linked aromatic polyimide polymer membrane into apolybenzoxazole membrane.

The self-cross-linkable aromatic polyimide polymer used for thepreparation of PBO membrane described in the present invention comprisea formula (I):

wherein X₁ and X₂ are selected from the group consisting of

and mixtures thereof, respectively; X₁ and X₂ are the same or differentfrom each other; Y₁—COOH is selected from the group consisting of

and mixtures thereof; Y₂—OR is selected from the group consisting of

and mixtures thereof, and —R— is selected from the group consisting of—H and a mixture of —H and —COCH₃, and —R′— is selected from the groupconsisting of

and mixtures thereof; n and m are independent integers from 2 to 500;the molar ratio of n/m is in a range of 1:1 to 1:20.

In the self-cross-linkable aromatic polyimide polymer in formula (I), X₁and X₂ may be selected from the group consisting of

and mixtures thereof. In the process of the invention, in formula (I)Y₁—COOH may be

In the process of the invention, the self-cross-linkable aromaticpolyimide polymer in formula (I) may include Y₂—OR that is selected fromthe group consisting of

and mixtures thereof.

In an embodiment of the invention, the self-cross-linkable aromaticpolyimide polymer comprising both hydroxyl functional groups andcarboxylic acid functional groups are selected from the group consistingof poly(3,3′,4,4′-diphenylsulfone tetracarboxylicdianhydride-3,5-diaminobenzoicacid-3,3′-dihydroxy-4,4′-diamino-biphenyl)polyimide derived from apolycondensation reaction of 3,3′,4,4′-diphenylsulfone tetracarboxylicdianhydride with a mixture of 3,5-diaminobenzoic acid and3,3′-dihydroxy-4,4′-diamino-biphenyl; poly(3,3′,4,4′-benzophenonetetracarboxylic dianhydride-pyromellitic dianhydride-3,5-diaminobenzoicacid-3,3′-dihydroxy-4,4′-diamino-biphenyl)polyimide derived from apolycondensation reaction of 3,3′,4,4′-benzophenone tetracarboxylicdianhydride and pyromellitic dianhydride with 3,5-diaminobenzoic acidand 3,3′-dihydroxy-4,4′-diamino-biphenyl; poly(3,3′,4,4′-benzophenonetetracarboxylic dianhydride-3,5-diaminobenzoicacid-3,3′-dihydroxy-4,4′-diamino-biphenyl)polyimide derived from apolycondensation reaction of 3,3′,4,4′-benzophenone tetracarboxylicdianhydride with 3,5-diaminobenzoic acid and3,3′-dihydroxy-4,4′-diamino-biphenyl;poly[2,2′-bis-(3,4-dicarboxyphenyl)hexafluoropropanedianhydride-3,5-diaminobenzoicacid-3,3′-dihydroxy-4,4′-diamino-biphenyl]polyimide derived from thepolycondensation reaction of2,2′-bis-(3,4-dicarboxyphenyl)hexafluoropropane dianhydride with amixture of 3,5-diaminobenzoic acid and3,3′-dihydroxy-4,4′-diamino-biphenyl;poly[2,2′-bis-(3,4-dicarboxyphenyl)hexafluoropropanedianhydride-2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane-3,5-diaminobenzoicacid] derived from a polycondensation reaction of2,2′-bis-(3,4-dicarboxyphenyl)hexafluoropropane dianhydride with amixture of 2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane and3,5-diaminobenzoic acid; poly[3,3′,4,4′-benzophenonetetracarboxylicdianhydride-2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane-3,5-diaminobenzoicacid] derived from a polycondensation reaction of3,3′,4,4′-benzophenonetetracarboxylic dianhydride with a mixture of2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane and3,5-diaminobenzoic acid; poly[4,4′-oxydiphthalicanhydride-2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane-3,5-diaminobenzoicacid] derived from a polycondensation reaction of 4,4′-oxydiphthalicanhydride with a mixture of2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane and3,5-diaminobenzoic acid; poly[3,3′,4,4′-diphenylsulfone tetracarboxylicdianhydride-2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane-3,5-diaminobenzoicacid] derived from a polycondensation reaction of3,3′,4,4′-diphenylsulfone tetracarboxylic dianhydride with a mixture of2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane and3,5-diaminobenzoic acid;poly[2,2′-bis-(3,4-dicarboxyphenyl)hexafluoropropanedianhydride-3,3′,4,4′-benzophenonetetracarboxylicdianhydride-2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane-3,5-diaminobenzoicacid] derived from a polycondensation reaction of2,2′-bis-(3,4-dicarboxyphenyl)hexafluoropropane dianhydride and3,3′,4,4′-benzophenonetetracarboxylic dianhydride with a mixture of2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane and3,5-diaminobenzoic acid; poly[4,4′-oxydiphthalicanhydride-2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane-3,3′-dihydroxy-4,4′-diamino-biphenyl-3,5-diaminobenzoicacid] derived from a polycondensation reaction of 4,4′-oxydiphthalicanhydride with a mixture of2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane,3,3′-dihydroxy-4,4′-diamino-biphenyl and 3,5-diaminobenzoic acid;poly[3,3′,4,4′-benzophenonetetracarboxylicdianhydride-2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane-3,3′-dihydroxy-4,4′-diamino-biphenyl-3,5-diaminobenzoicacid] derived from a polycondensation reaction of3,3′,4,4′-benzophenonetetracarboxylic dianhydride with a mixture of2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane,3,3′-dihydroxy-4,4′-diamino-biphenyl, and 3,5-diaminobenzoic acid.

In another embodiment of the invention, the self-cross-linkable aromaticpolyimide polymer comprising both hydroxyl functional groups andcarboxylic acid functional groups is selected from the group consistingof poly(3,3′,4,4′-diphenylsulfone tetracarboxylicdianhydride-3,5-diaminobenzoicacid-3,3′-dihydroxy-4,4′-diamino-biphenyl)polyimide derived from apolycondensation reaction of 3,3′,4,4′-diphenylsulfone tetracarboxylicdianhydride with a mixture of 3,5-diaminobenzoic acid and3,3′-dihydroxy-4,4′-diamino-biphenyl; poly(3,3′,4,4′-benzophenonetetracarboxylic dianhydride-pyromellitic dianhydride-3,5-diaminobenzoicacid-3,3′-dihydroxy-4,4′-diamino-biphenyl)polyimide derived from apolycondensation reaction of 3,3′,4,4′-benzophenone tetracarboxylicdianhydride and pyromellitic dianhydride with 3,5-diaminobenzoic acidand 3,3′-dihydroxy-4,4′-diamino-biphenyl;poly[2,2′-bis-(3,4-dicarboxyphenyl)hexafluoropropanedianhydride-3,5-diaminobenzoicacid-3,3′-dihydroxy-4,4′-diamino-biphenyl]polyimide derived from thepolycondensation reaction of2,2′-bis-(3,4-dicarboxyphenyl)hexafluoropropane dianhydride with amixture of 3,5-diaminobenzoic acid and3,3′-dihydroxy-4,4′-diamino-biphenyl; poly[3,3′,4,4′-diphenylsulfonetetracarboxylicdianhydride-2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane-3,5-diaminobenzoicacid] derived from a polycondensation reaction of3,3′,4,4′-diphenylsulfone tetracarboxylic dianhydride with a mixture of2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane and3,5-diaminobenzoic acid; poly[3,3′,4,4′-benzophenonetetracarboxylicdianhydride-2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane-3,3′-dihydroxy-4,4′-diamino-biphenyl-3,5-diaminobenzoicacid] derived from a polycondensation reaction of3,3′,4,4′-benzophenonetetracarboxylic dianhydride with a mixture of2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane,3,3′-dihydroxy-4,4′-diamino-biphenyl, and 3,5-diaminobenzoic acid.

In another embodiment of the invention, the self-cross-linked aromaticpolyimide polymer form from the self-cross-linkable aromatic polyimidepolymer described comprises a plurality of repeating units of formula(II):

wherein X₁ and X₂ are selected from the group consisting of

and mixtures thereof, respectively; X₁ and X₂ are the same or differentfrom each other; Y₁—CO— is selected from the group consisting of

and mixtures thereof; Y₂—O— is selected from the group consisting of

and mixtures thereof, and —R′— is selected from the group consisting of

and mixtures thereof; Y₂—OR is selected from the group consisting of

and mixtures thereof, and —R— is selected from the group consisting of—H and a mixture of —H and —COCH₃, and —R′— is selected from the groupconsisting of

and mixtures thereof; n′, n″, m′, m″, p, and p′ are independent integersfrom 2 to 500; the molar ratio of n′/(m′+p) is in a range of 1:1 to1:20; the molar ratio of n″/(m″+p′) is in a range of 1:1 to 1:20. Inembodiments of the invention in formula (II), Y₁—CO— may be

Y₂—O— may be selected from the group consisting of

and mixtures thereof, and Y₂—OR may be selected from the groupconsisting of

and mixtures thereof. In an embodiment of the invention, thepolybenzoxazole polymer comprises repeating units of a formula (III),wherein said formula (III) is:

wherein X₁ is selected from the group consisting of

and mixtures thereof; wherein X₃ is selected from the group consistingof

and mixtures thereof; wherein X₄ is selected from the group consistingof

and mixtures thereof; Y₁ is selected from the group consisting of

and mixtures thereof; Y₂ is selected from the group consisting of

and mixtures thereof, and —R′— is selected from the group consisting of

and mixtures thereof; o and q are independent integers from 2 to 500. Inan embodiment of the invention, X₁ may be selected from the groupconsisting of

and mixtures thereof; X3 may be selected from the group consisting of

X₄ may be selected from the group consisting of

Y₁ may be

andY₂ may be selected from the group consisting of

and mixtures thereof.

The method of preparing the membranes of the invention may furthercomprise application of a high permeability material to a surface ofsaid polybenzoxazole membrane wherein said high permeability material isselected from the group consisting of a polysiloxane, a fluoro-polymer,a thermally curable silicone rubber, or a UV radiation curable epoxysilica. The polybenzoxazole membrane may be fabricated into a flatsheet, tube or hollow fiber membrane or other form as known to one ofskill in the art.

The invention also involves preparation of a polybenzoxazole membraneprepared by any of the preceding embodiments.

In another embodiment of the invention is provided a process forseparating at least one gas from a mixture of gases comprising providinga self-cross-linked aromatic polyimide membrane of formula (III)comprising wherein said formula (III) is:

wherein X₁ is selected from the group consisting of

and mixtures thereof; wherein X₃ is selected from the group consistingof

and mixtures thereof; wherein X₄ is selected from the group consistingof

and mixtures thereof; Y₁ is selected from the group consisting of

and mixtures thereof; Y₂ is selected from the group consisting of

and mixtures thereof, and —R′— is selected from the group consisting of

and mixtures thereof; o and q are independent integers from 2 to 500contacting the mixture of gases to one side of the polybenzoxazolemembrane of formula (III) to cause at least one gas to permeate saidmembrane; and removing from an opposite side of the polybenzoxazolemembrane of formula (III) a permeate gas composition comprising aportion of said at least one gas that permeated said membrane. Informula (III), X₁ may be selected from the group consisting of

and mixtures thereof, X3 may be selected from the group consisting of

X₄ may be selected from the group consisting of

Y₁ may be

andY₂ may be selected from the group consisting of

and mixtures thereof.

The mixture of gases may be any mixture of gases that may be separatedby a membrane. The mixture of gases may be a mixture of carbon dioxideand methane, a mixture of hydrogen and methane, or a mixture of heliumand methane as well as other gases found in natural gas. The mixture ofgases may comprise a mixture of at least one volatile organic compoundand at least one atmospheric gas. The mixture of gases may comprisenitrogen and hydrogen. The mixture of gases treated by the membranes ofthis invention may comprise a mixture of carbon dioxide, oxygen,nitrogen, water vapor, hydrogen sulfide, helium and methane.

In some embodiments of the invention, the membrane may comprise aspecies that adsorbs strongly to at least one gas. In some embodimentsof the invention, the mixture of gases comprises a mixture of paraffinsand olefins.

Another embodiment of the invention involves a process for separation ofliquid mixtures by pervaporation comprising contacting said liquidmixture with a polybenzoxazole membrane of formula (III) comprisingwherein said formula (III) is:

wherein X₁ is selected from the group consisting of

and mixtures thereof; wherein X₃ is selected from the group consistingof

and mixtures thereof; wherein X₄ is selected from the group consistingof

and mixtures thereof; Y₁ is selected from the group consisting of

and mixtures thereof; Y₂ is selected from the group consisting of

and mixtures thereof, and —R′— is selected from the group consisting of

and mixtures thereof; o and q are independent integers from 2 to 500contacting the mixture of liquids to one side of the polybenzoxazolemembrane of formula (III) to cause at least one gas to permeate saidmembrane; and removing from an opposite side of said polybenzoxazolemembrane of formula (III) a permeate liquid composition comprising aportion of said at least one liquid that permeated said membrane.

The liquid mixture may comprise water and one or more organic compoundsselected from the group consisting of alcohols, phenols, chlorinatedhydrocarbons, pyridines, and ketones and the process involves separationof water from the one or more organic compounds. The liquid mixture maycomprise sulfur-containing molecules in a hydrocarbon stream such asnaphtha. The membranes used in the process can be used to remove suchsulfur-containing molecules from diesel or gasoline products. In anotherembodiment of the invention, the liquid mixture may comprise a mixtureof isomers of organic compounds. The liquid mixture may comprise amixture selected from the group consisting of: ethylacetate-ethanol,diethylether-ethanol, acetic acid-ethanol, benzene-ethanol,chloroform-ethanol, chloroform-methanol, acetone-isopropylether,allylalcohol-allylether, allylalcohol-cyclohexane, butanol-butylacetate,butanol-1-butylether, ethanol-ethylbutylether, propylacetate-propanol,isopropylether-isopropanol, methanol-ethanol-isopropanol, andethylacetate-ethanol-acetic acid.

In another embodiment of the invention, the liquid mixture comprises adilute ethanol solution and where said process increases an ethanolconcentration in said liquid mixture.

1. A process for separating at least one gas from a mixture of gasescomprising providing a polybenzoxazole membrane of formula (III)comprising wherein said formula (III) is:

wherein X₁ is selected from the group consisting of

and mixtures thereof; wherein X₃ is selected from the group consistingof

and mixtures thereof; wherein X₄ is selected from the group consistingof

and mixtures thereof; Y₁ is selected from the group consisting of

and mixtures thereof; Y₂ is selected from the group consisting of

and mixtures thereof, and —R′— is selected from the group consisting of

and mixtures thereof; o and q are independent integers from 2 to 500contacting the mixture of gases to one side of the polybenzoxazolemembrane of formula (III) to cause at least one gas to permeate saidmembrane; and removing from an opposite side of said polybenzoxazolemembrane of formula (III) a permeate gas composition comprising aportion of said at least one gas that permeated said membrane.
 2. Theprocess of claim 1 wherein in said formula (III), X₁ is selected fromthe group consisting of

and mixtures thereof.
 3. The process of claim 1 wherein in said formula(III), X3 is selected from the group consisting of


4. The process of claim 1 wherein in said formula (III) X₄ is selectedfrom the group consisting of


5. The process of claim 1 wherein in said formula (III), Y₁ is


6. The process of claim 1 wherein Y₂ is selected from the groupconsisting of

and mixtures thereof.
 7. The process of claim 1 wherein said mixture ofgases comprises a mixture of carbon dioxide and methane.
 8. The processof claim 1 wherein said mixture of gases comprises a mixture of hydrogenand methane.
 9. The process of claim 1 wherein said mixture of gasescomprises a mixture of helium and methane.
 10. The process of claim 1wherein said mixture of gases comprises a mixture of at least onevolatile organic compound and at least one atmospheric gas.
 11. Theprocess of claim 1 wherein said mixture of gases comprises nitrogen andhydrogen.
 12. The process of claim 1 wherein said mixture of gasescomprises a mixture of carbon dioxide, oxygen, nitrogen, water vapor,hydrogen sulfide, helium and methane.
 13. The process of claim 1 whereinsaid self-cross-linked aromatic polyimide polymer membrane comprises aspecies that adsorbs strongly to at least one gas.
 14. The process ofclaim 1 wherein said mixture of gases comprises a mixture of paraffinsand olefins.
 15. A process for separation of liquid mixtures bypervaporation comprising contacting said liquid mixture with apolybenzoxazole membrane of formula (III) comprising wherein saidformula (III) is:

wherein X₁ is selected from the group consisting of

and mixtures thereof; wherein X₃ is selected from the group consistingof

and mixtures thereof; wherein X₄ is selected from the group consistingof

and mixtures thereof; Y₁ is selected from the group consisting of

and mixtures thereof; Y₂ is selected from the group consisting of

and mixtures thereof, and —R′— is selected from the group consisting of

and mixtures thereof; o and q are independent integers from 2 to 500contacting the mixture of liquids to one side of the polybenzoxazolemembrane of formula (III) to cause at least one gas to permeate saidmembrane; and removing from an opposite side of said polybenzoxazolemembrane of formula (III) a permeate liquid composition comprising aportion of said at least one liquid that permeated said membrane. 16.The process of claim 15 wherein said liquid mixture comprises water andone or more organic compounds selected from the group consisting ofalcohols, phenols, chlorinated hydrocarbons, pyridines, and ketones. 17.The process of claim 15 wherein said liquid mixture comprisessulfur-containing molecules in a hydrocarbon stream.
 18. The process ofclaim 15 wherein said liquid mixture comprises a mixture of isomers oforganic compounds.
 19. The process of claim 15 wherein said liquidmixture comprises a mixture selected from the group consisting of:ethylacetate-ethanol, diethylether-ethanol, acetic acid-ethanol,benzene-ethanol, chloroform-ethanol, chloroform-methanol,acetone-isopropylether, allylalcohol-allylether,allylalcohol-cyclohexane, butanol-butylacetate, butanol-1-butylether,ethanol-ethylbutylether, propylacetate-propanol,isopropylether-isopropanol, methanol-ethanol-isopropanol, andethylacetate-ethanol-acetic acid.
 20. The process of claim 15 whereinsaid liquid mixture comprises a dilute ethanol solution and where saidprocess increases an ethanol concentration in said liquid mixture.